Field of the Invention
The invention relates to the field of devices for converting an optical quantity into an electrical quantity.
In devices transmitting a video signal, for example a television signal, by optical means, for example by means of an optical fibre, there is, at one end of the optical fibre, or generally of the light channel in which the signal-carrying optical wave propagates, a converter which converts an optical quantity, for example optical energy, into an electrical quantity, for example a voltage or a current. The invention is applicable to the case in which this converter is a PIN photodiode.
A PIN photodiode is equivalent, when it is suitably biased by virtue of a positive voltage applied between its cathode (N junction) and its anode (P junction), to a current generator delivering a current i(t) expressed by the equation: EQU i(t)=S P(t).
In this equation,
i(t) is the instantaneous strength of the current, PA1 S is the sensitivity of the PIN photodiode expressed PA1 sed in amps per watt, PA1 P(t) is the value of the instantaneous optical PA1 power received by the PIN photodiode. PA1 S.sub.1 =M S, PA1 S being the sensitivity of a conventional PIN photodiode, PA1 M being a coefficient lying between 1 and .congruent.20. The value of the multiplication factor M is a function of the value of the bias voltage of the avalanche photodiode. By exploiting this property, it is possible by virtue of an electrical feedback loop to slave the value of M to the value of the optical power incident on the photodetector stage.
Since the incident optical power generally varies from a low value to a very low value, the same applies to the value of the photodetected current and it may be imagined that it is necessary to amplify the latter extremely carefully. Such quality of amplification is achieved by making use of a so-called transimpedance-type preamplifier stage obtained by wiring up a resistor Rg between the output and the e(-) input of an amplifier of gain Gv having quite exceptional characteristics: A gain-bandwidth product greater than several tens of GHz in many applications.
A known embodiment of such a photodetection stage is depicted in FIG. 1.
This figure shows one end 1 of an optical fibre transmitting a light wave modulated by a video-frequency signal. The modulated light energy is received by a PIN photodiode 2 which includes an anode (P junction) 3, an intrinsic region 4 and a cathode 5. The PIN photodiode 2 is, in a known manner, reverse-biased, that is to say that a positive voltage is applied to its cathode 5. Under these conditions, the PIN photodiode 2 delivers a current which flows from its cathode 5 to its anode 3. This current is proportional to the value of the incident optical power. The proportionality ratio represents the sensitivity of the PIN photodiode 2.
FIG. 2 shows curves representing the value of the photodetected current I.sub.ph as a function of the reverse-bias voltage V.sub.p. Each of the curves C1 . . . C5 in FIG. 2 is the curve representing the value of the photodetected current for a constant photoelectric power as a function of the bias voltage. When this power varies and when the PIN photodiode 2 is biased, for example by a value V.sub.po, the operating point of the PIN photodiode 2 is shifted along a vertical line portrayed in FIG. 2 by a dotted line passing through the V.sub.po, value and parallel to the I.sub.ph axis representing the currents as a function of the incident power.
The current I.sub.ph is amplified by a transimpedance preamplifier stage 6 having a negative input 7, a positive input 8, an output 9 and a feedback resistor Rg 10 connected between the output 9 and the negative input 7.
It is known that the value of the noise current ieff (Rg) due to Rg, appearing on the e(-) input 7 of the transimpedance preamplifier stage 6, is given by the equation: ##EQU1##
It is easy to imagine the advantage of being able, for signal-to-noise ratio reasons, to use a high to very high value of the feedback resist or Rg.
Due to the feedback phenomenon to which the resistor Rg 10 is subjected, the apparent value of the feedback resistor Rg 10 seen by the PIN photodiode 2 is extremely low.
This apparent resistance Rapp is expressed by the equation: ##EQU2##
In this equation, G is the gain of the transimpedance preamplifier stage 6.
The negative input 7 is thus equivalent to a virtual earth and all the current i(t) is diverted to the feedback resistor Rg 10. Thus, the output voltage V.sub.s (t) is of the form: EQU V.sub.s (t)=i(t)Rg=S P.sub.h (t)Rg.
If the average incident optical power P.sub.h (t) reaches a very high value, which would be the case for example for a detector stage located at one end of a very short optical fibre, between emitter and receiver, or else at a very short distance from a repeater, the value of V.sub.s (t)=S P.sub.h (t) Rg may reach a very high value. Under these conditions, the transimpedance preamplifier stage 6 saturates and no longer operates in a linear fashion.
This problem of saturation of the transimpedance preamplifier stage 6 is known and various solutions have been used to remedy it.
A first known embodiment aimed at avoiding the saturation of the transimpedance preamplifier stage 6 consists in using a feedback resistor Rg 10 of low value in such a way that V.sub.s (t)max=i(t)max Rg does not exceed the maximum permissible V.sub.s value. This solution is offset by a deterioration in the intrinsic signal-to-noise ratio of the optical receiver because of the increase in the term ##EQU3##
In this expression, .alpha. is a coefficient of proportionality.
A second known embodiment aimed at avoiding the saturation of the transimpedance preamplifier stage 6 consists in inserting an attenuator in the optical link in order to prevent P(t) from exceeding the maximum permissible value P(t) maximum. This individual optimization constraint for each link is currently no longer accepted by users.
This embodiment requires the latter to measure or calculate the average incident optical power in order to determine the value of the attenuator to be inserted.
Finally, a third known solution consists in replacing the PIN photodiode 2 with an avalanche photodiode. An avalanche photodiode is equivalent to a PIN photodiode which would be characterized by a sensitivity:
This effective solution has the drawback of being extremely expensive because, on the one hand, of the cost of an avalanche diode compared to the cost of a PIN photodiode and, on the other hand, of the cost of the electronic circuits for slaving the avalanche diode.
The purpose of the present invention is to provide a transimpedance preamplifier stage using a PIN detection photodiode whose average output current remains below the current causing saturation of the transimpedance preamplifier stage, this being so without decreasing the value of the signal/noise ratio of this transimpedance preamplifier stage. By this is meant that, for the same theoretical signal/noise ratio value, the incident-light power dynamics of the device according to the invention are superior to those of PIN photodiode devices according to the prior art. Put another way, for the same feedback resistor Rg of the transimpedance preamplifier stage, the light-power reception dynamics of the device according to the invention are superior to those of PIN photodiode devices of the prior art.
Thus, the device according to the invention may be employed, without any special precautions or adjustments, at one end of an optical fibre whose other end receives light power from an emitter, or from a repeater, this being the case virtually whatever the distance between the emitter or the repeater and the device according to the invention. In particular, it will no longer be necessary, as in the first embodiment of the prior art, to adjust the value of the feedback resistor RG, in order to decrease it when the distance from the source decreases, thereby paradoxically increasing the noise of the stage and therefore decreasing the signal/noise ratio. It will not be necessary either, as in the second embodiment of the prior art, to decrease the received light power by means of an attenuator when the average light power received by the PIN photodiode increases, for example because of the shortness of the optical line between the final repeater and the device according to the invention.
In short, the use of the PIN photodiode photodetector device according to the invention relieves the user of any worries about matching the device to the average level of the optical power received locally. The matching takes place automatically.
In order to maintain the average value of the modulation current photodetected by the PIN photodiode, provision is made according to the present invention to decrease the sensitivity of the PIN photodiode above a certain threshold of average received light power, P.sub.h threshold.
Above this threshold P.sub.h threshold the average current generated by the PIN photodiode remains constant.
The invention therefore relates to a device for detecting a signal carried by a modulated light wave, the device receiving the wave on a PIN photodiode having two electrodes, an anode and a cathode, as well as an intrinsic part, at least one of the electrodes of the PIN photodiode being connected to biasing means, the PIN photodiode delivering, when it is reverse-biased by a positive voltage applied between its cathode and its anode, a photodetection current i(t) proportional to the power of the incident light wave, which detection device is characterized in that the means for biasing the PIN photodiode are capable of applying two bias states to the PIN photodiode, a first bias state and a second bias state, the bias being automatically established in the first or in the second state depending on the average received light power; in the first state, corresponding to average received light powers below a threshold value P.sub.h threshold, the photodiode is reverse-biased by the biasing means; in the second state, corresponding to average received light powers greater than the threshold value P.sub.h threshold, the photodiode is forward-biased by the said biasing means, the average photodetection current delivered by the photodiode then being constant.